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Early 650ss rotor and stator replacement

Submitted by kit_lewis on Wed, 06/03/2024
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Heavy Twins

Hello. I’m trying to recommission an old 650ss which has stood idle for over 40 years. The engine number shows it to be from Sept 1962 but from the 1963 model year. 

On stripping the primary chain case I was confronted with one of the old RM 15 70mm rotor and stator sets which are both ‘shot’ and the rotor magnets are very feeble. Time for a new set! 

The new 74mm Lucas replacements look lovely and shiny in the box right up until the point where the new rotor doesn’t actually fit the crank.

This came as something of a surprise but then again this is my first Norton and the learning curve is somewhat vertical at present……

OK. Help required.

My crankshaft at the relevant position measures .766 inches

The old rotor had been somewhat ‘adapted’ to fit the crank and a sleeve seems to have been inserted and ‘peened’ into place. Unsurprisingly it is a good fit to the crank. 

The new rotor is much smaller and measures in at .724 inches. No way does this fit. 

I read that you cannot get 70mm rotors anymore even if I could get the stator rewound.

So what are my options here? Machine the rotor to fit the crank seems about the only solution here, but also somewhat excessive. I’ve scoured the compendium and searched Roadholder but to no avail. Perhaps I’m not looking in the right place or have my search terms muddled?

Photos attached to show the old stator and rotor but you all know what they look like. I’d be grateful for someone to point me in the right direction or explain what I should do next…..

Cheers

Kit 

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Submitted by david_cooper on Thu, 07/03/2024

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Standard rotors are advertised as 19mm, which is what you have (19mm = 0.748mm).  Someone has repaired the tip of your crank, possibly because of damage to the original rotor keyway.  Metal spray, or a sleeve, perhaps?  It looks like you already know what you're going to have to do: bore the new rotor out on a lathe.  It's a stop-start cut, because of the keyway in the rotor...I don't know how important that is.

The Lucas manual describes the newer alternator rotors were suitable for shafts up to 1" = 25.4mm.  Does the new one have a bush that might be replaced?

Submitted by kit_lewis on Thu, 07/03/2024

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Thanks. I did wonder why the original rotor had been so extensively modified/bodged/fettled. 
I hadn’t considered that the crank itself might have been repaired. This is a proper dog’s breakfast of an engine! 

Submitted by alan_osborn on Thu, 07/03/2024

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The Lucas manual describes the newer alternator rotors were suitable for shafts up to 1" = 25.4mm.  Does the new one have a bush that might be replaced?   This I find a very interesting statement. LUCAS MANUAL? now is this a word from the 1980s Real Lucas or a modern spoof? I say spoof as every 74mm modern (made in far East) rotor I have seen is 19mm hole! Especially as the Wipac rotor on the lightweights was/is 1". And boring the 19mm out to 25mm cannot be done with safety.

I have some good 70mm rotors. But rewinding the stator I have NOT found, if any one does find someone to do this let me know I have some work for him.

If you bore out the (what should be 19mm) rotor will its 'security' be compromised? Where are you with any future Guarantee for the rotor?

I would reckon any guarantee might disappear with the first cut from the borer! However if I don’t do that then I have to replace or grind the crank and I know which will be cheaper! Hopefully rotor integrity shouldn’t be compromised? Anyone tell me different? 

Thanks David  That’s fascinating stuff and confirms it’s an RM15. It looks like the centre hole was bored out and a new sleeve inserted to suit the repaired crank. 

I’ve found a CNC engineering company to take on the work on the new rotor so let’s see what happens. They are confident it should not damage the integrity of the rotor. Time, and a few thousand revs will tell.

 

Submitted by alan_hesslewood on Fri, 08/03/2024

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Does anyone know what different widths the Lucas rotors were made in?

(Sorry, slight thread hijack but related)

Regards, Al.

Submitted by alan_hesslewood on Sat, 09/03/2024

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I’ve had another look David but still can’t see any dimensions for the rotor widths, only the diameters. Anybody know the different widths made?

Submitted by alan_osborn on Sat, 09/03/2024

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There is only the one now. In the dim past of RM18 and ANY previouse RM..there were several widths, but it is all academic now. If you have an early rotor with enough magnetism to be usable then you will have to space in/out to suit the stator and crank and primary cover with washers-spacers etc.   Have fun.

Submitted by robert_tuck on Sat, 09/03/2024

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I ordered a new complete alternator from RGM , it was the high output type .It would have been helpfull if the supplier had advised that a spacer might be necessary and that thicker studs would be needed ,and they could have supplied studs too .  Why not?.

Hi Robert,

Does the rotor require a spacer behind and therefore is it narrower than the previous one? What are the measurements of both rotors? (No one seems to know what widths have been available).

Regards, Al.

 



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